MSO: get TTAGs from trigger channel instead of first readout channel.

Now read out the actual trigger channel after the setting process, and
use this one as default channel for waveform information (WAV:COUNt?
polling in the event loop) and for the TTAGs information.
Since the TTAG are trigger time difference, I'm not sure what it looks
like on non-trigger channels (the trigger channel is not always the
first in the readout list).

The default channel setting is now done at the start of each run (ie
after the data fetch and writing from any channel occured at the end of
the previous run).

Improves the readout settings that was changed in #5911603e
9 jobs for MSO_dump_ttags in 3 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Environment
passed #461033
check_conda

00:01:48

passed #461032
check_env

00:00:28

 
  Static
passed #461034
pylint_DG82x

00:00:29

passed #461035
pylint_MSOS

00:00:27

passed #461036
pylint_SSP816x

00:00:29

passed #461037
pylint_TandD_XML

00:00:30

 
  Test
passed #461038
test_MSOS

00:00:24

passed #461039
test_TandD_XML

00:00:23

 
  Run
passed #461040
run_TandD_plot

00:00:24